1. Field of the Invention
The present invention relates in general to the field of electronics, and more specifically to a system and method that provides multi-mode flyback control for a switching power converter.
2. Description of the Related Art
Many electronic systems utilize switching power converters to efficiently convert power from one source into power useable by a device (referred to herein as a “load”). Some facilities, such as homes and buildings, include light source dimming circuits (referred to herein as a “dimmer”). Dimmers control the power delivered to a load, such as a lamp having one or more light sources. Dimming saves energy and, in a lighting installation, also allows a user to adjust the intensity of the light source to a desired level.
A switching power converter is controlled by a controller. For systems that are not intended for use with dimmers, the controller provides power factor correction for displacement and distortion so that the phases of an alternating current (AC) supply voltage and an average input current to the switching power converter are approximately in-phase (power factor displacement) and the current shape follows the shape of the supply voltage (power factor distortion). For systems that include dimmers and constant current loads, the controller controls the switching power converter to provide a constant, average input current to the load without regard to power factor correction.
FIG. 1 depicts an electronic system 100 that converts power from voltage supply 102 into power usable by load 104. Load 104 is a constant current load that includes, for example, one or more light emitting diodes (LEDs). A controller 106 controls the power conversion process. Voltage source 102 can be any type of voltage source such as a public utility supplying a 60 Hz/110 V input voltage VN in North America or a 50 Hz/220 V input voltage VIN in Europe or the People's Republic of China, or a DC voltage source supplied by a battery or another switching power converter. The dimmer 103 phase cuts the supply voltage VSUPPLY, and the full-bridge rectifier 105 rectifies the phase-cut supply voltage to generate a rectified phase-cut input voltage VIN to the flyback-type switching power converter 110. In at least one embodiment, the dimmer 103 is a triac-based dimmer.
The controller 106 provides a pulse width modulated (PWM) control signal CS0 to current control switch 108 in a flyback-type, switching power converter 110 to control the conversion of input voltage VIN into a primary-side voltage VP and secondary voltage VS. When the switch 108 is non-conductive, i.e. “off”, the primary voltage VP is N times the secondary voltage VS, i.e. VP=N·VS, and “N” is a ratio of turns in the primary-winding 114 to the turns in the secondary-winding 118. The switch 108 is, for example, a field effect transistor (FET). When control signal CS0 causes switch 108 to conduct, a primary-side current iIN flows into a primary-winding 114 of transformer 116 to energize the primary-winding 114. When switch 108 conducts, the diode 120 is reverse biased, and the secondary-side current iS is zero. When control signal CS0 opens switch 108, the primary voltage VP and secondary voltage VS reverse the indicated polarities, and diode 120 is forward biased. When diode 120 is forward biased, the secondary side current iS flows through the diode 120 to charge capacitor 122 so that an approximately constant current and direct current (DC) voltage VLD is provided to the load 104. Since the dimmer 103 is present, the controller 106 controls the input current iIN as a constant current.